The invention regards an apparatus and method to maintain constant fluid circulation during the entire drill process.
Drilling fluids (“mud”) are complex mixtures based on water or oil used to stabilize the borehole when drilling for oil, and to transport solid material, cuttings, to the surface. Water based drilling fluid is treated, cuttings are removed by separation, and the water is re-circulated to a large extent. Oil based drilling fluids are disposed of, re-circulated after treatment or used for heating oil.
The drilling fluid is circulated continuously. When a new section of drilling pipes is connected to (“makeup”) or disconnected from (“breakout”) the top of a string of pipes, the circulation of drilling fluid must be maintained. If circulation stops, the drilling fluid will settle, acquire a jelly-like form, and require a large pressure to resume the circulation. This may lead to a punctured formation, which collapses, and large losses of drilling fluid into the formation.
When drilling through reservoirs or formations having small margins between fracturing gradient and pore pressure, it is of great importance that the pressure at the bottom of the bore, measured by Equivalent Circulation Density (ECD), is maintained within small margins. Starting and stopping mud pumps creates pressure changes which may exceed these margins.
ECD (Equivalent Circulation Density) depends on factors like:                mud weight        type and composition of mud        temperature        circulation rate        gel strength.        
In a typical drilling operation, drill pipes of different sizes are used depending on several factors. The drill pipes are typically 9.3 m long, and are threaded together into stands, each stand having three drill pipes. When inserting or withdrawing a string of drill pipes, stands or single drill pipes must be connected to or disconnected from the string of drill pipes. This means that during circulation, the mud pumps must be stopped and started when pipes are connected or disconnected. Start and stop causes pressure changes mentioned above, resulting in formation fractures and losses. This constitutes a large risk regarding well security, and additionally a large economic cost, as it may lead to loss of the entire section or the entire well.
All the factors and conditions above are important in wells classified as:                HTHP—High Temperature High Pressure        UBD—Under Balanced Drilling.        ERD—Extended Reach Drilling        
Thus, it is important to maintain continuous circulation of drilling fluid during the entire drilling operation.
GB 2.427.217-A (corresponding to WO A1 2006133826, ENI) discloses a short tubular section (a valve section) having two valves. The section is threaded at both ends, and is included in a drill string. One valve is connected to an external coupling, and is used for opening or closing a radial inlet for drilling fluid. The second valve is operated by pressure, and is used for opening or closing a top inlet for flow of drilling fluid axially along the valve section. A larger pressure at the top inlet opens the axial valve, and a larger pressure at the radial side inlet closes the axial valve. When pipes are connected or disconnected, the top inlet is closed, and drilling fluid circulates through the radial inlet. The radial inlet may, if needed, be secured by a plug. In a preferred embodiment, one or both valves are flapper valves. One problem with this device, is that the pressures required to open or close the axial pressure activated valve may exceed the permitted pressure limits. A second problem with this device, is that any pressure operated valve needs a surface on which the pressure may work. A surface extending radially into the central bore to allow opening or closing of the axial valve can make it difficult or impossible to pass certain tools through the central bore.
U.S. Pat. No. 7,107,875 B2 (Haugen et al) also discloses an apparatus that permits sections of pipes to be connected to or disconnected from a string of pipe during a drilling operation. The apparatus allows for the continuous fluid flow to and through the tubular string during makeup and breakout, Rotation and axial movement of the tubular string is alternately provided by a top drive and a rotary drive. Continuous fluid flow into the tubular string is provided through the circulation device and alternately through the main bore once a connection is made between an upper pipe connected to the top drive mechanism and the string of pipe. The circulation device of U.S. Pat. No. 7,107,875 comprises an upper chamber with an opening for the upper pipe, a lower chamber with an opening for the string of pipes, and a gate apparatus between the upper and lower chambers. The gate apparatus is open during connection and disconnection of pipes, and closed when a new section is prepared for or removed from the string of pipes. One problem with this device is that it requires two drives, i.e. a top drive and a rotary drive. Another problem is that drill pipes rotates in holes through the roof of the upper chamber and in the floor of the lower chamber when an upper stand or section of drill pipes are connected to or disconnected from a lower string of pipes, and when the chamber is pressurized. The need for rotating pipes in apertures as described above makes it difficult to adapt the device for high-pressure applications.
WO 2005/080745 (Statoil) discloses a hollow, cylindrical body (a valve section) having a mud inlet in a sidewall, and a valve which in a first position closes the side inlet and opens for fluid flow between the ends of the body, and which in a second position prevents fluid flow between the ends of the body and permits a flow from the side inlet to the lower end of the body. A major problem with this device is an increased risk for pressure shocks when the valve element is rotated. A state in which the valve is briefly allowing a lesser rate of fluid may cause a shock in high pressure applications. Another problem which may occur in high pressure applications, is that a high axial pressure may suddenly switch the valve to a position where the flow is along the central bore of the section which is to be included in the drill string. The problem is that special care must be taken to avoid the situations above, and the resulting pressure shocks in the drill string. This means the device will have problems in high pressure applications, either through an increased risk for pressure pulses, or through expensive correction means.
WO 2005/019596 (Coupler Developments) discloses a cylindrical body (diverter sub) having a valve which can be rotated to a first position in order to open for fluid flow through a top inlet and axially through the central bore, and to a second position to open for fluid flow through a radial side inlet. FIG. 11 shows an embodiment having two valves, in which an upper valve opens to drain the diverter sub or upper tubular before disconnecting it, and in which a lower valve closes the stream of drilling fluid through a top inlet while simultaneously opening for a flow of drilling fluid through a side inlet. The device in WO 2005/019596 can be designed with a bore of substantially equal diameter along the length thereof to allow passage of wireline tools. The problems of the device having one valve are similar to the problems associated with the Statoil device. The second valve of FIG. 11 is provided to drain the upper section of drill pipe. Thus, it is a valve of a different making provided for another purpose than the second valve of the present invention. Specifically, it does not provide a solution to the problems associated with high pressure applications.
NO168262 B (Hydril) shows a diverter sub that can be used as a blow out preventer. This device provides an apparatus of a different design to solve a different problem, and is cited as technical background.
The international company National Oilwell Varco has developed a system enabling continuous circulation of drilling fluid when sections of drill pipes are connected to, or disconnected from, a string of drill pipes This system is called “CCS-9-5k Continuous Circulation System”, and is a commercially available product used in the international market. This system is quite complicated, and may require a rebuild of part of the drilling floor and the areas around it. This is costly, a logistic challenge, and a time consuming process. The system replaces, among other things, the BlowOut Preventer (BOP), the iron roughneck and a possible snubbing unit. The main problems of this device are complexity and the required modifications of the rig.
US A1 20060254822, U.S. Pat. No. 3,298,325 and U.S. Pat. No. 2,158,356 disclose solutions for maintaining continuous fluid circulation in a well when drill pipes are connected to or disconnected from a string of drill pipes. These are cited as technical background.